Rubbers in particular those containing repeating units derived from isoolefins are industrially prepared by carbocationic polymerization processes. Of particular importance is butyl rubber which is a elastomer of isobutylene and a smaller amount of a multiolefin such as isoprene.
The carbocationic polymerization of isoolefins and its elastomerization with multiolefins is mechanistically complex. The catalyst system is typically composed of two components: an initiator and a Lewis acid such as aluminum trichloride which is frequently employed in large scale commercial processes.
Examples of initiators include proton sources such as hydrogen halides, carboxylic acids and water.
During the initiation step, the isoolefin reacts with the Lewis acid and the initiator to produce a carbenium ion which further reacts with a monomer forming a new carbenium ion in the so-called propagation step.
The type of monomers, the polymerization temperature as well as the specific combination of Lewis acid and initiator affects the chemistry of propagation and thus monomer incorporation into the growing polymer chain.
In addition to that the type of diluent or solvent and its polarity was found to have a significant influence on the polymerization and the final polymer product as well.
Industry has generally accepted widespread use of a slurry polymerization process to produce butyl rubber, polyisobutylene, etc. in methyl chloride as diluent. Typically, the polymerization process is carried out at low temperatures, generally lower than −90° C. Methyl chloride is employed for a variety of reasons, including that it dissolves the monomers and aluminum chloride catalyst but not the polymer product. Methyl chloride also has suitable freezing and boiling points to permit, respectively, low temperature polymerization and effective separation from the polymer and unreacted monomers. The slurry polymerization process in methyl chloride offers a number of additional advantages in that a polymer concentration of up to 40 wt.-% in the reaction mixture can be achieved, as opposed to a polymer concentration of typically at maximum 20 wt.-% in solution polymerizations. An acceptable relatively low viscosity of the polymerization mass is obtained enabling the heat of polymerization to be removed more effectively by surface heat exchange. Slurry polymerization processes in methyl chloride are used in the production of high molecular weight polyisobutylene and isobutylene-isoprene butyl rubber polymers.
It is known from EP1572766 A to use hydrofluorocarbons as diluents for the preparation of copolymers of an isoolefin, preferably isobutylene, and a multiolefin, preferably a conjugated diene, more preferably isoprene.
However, at the low temperatures applied the rate of polymerization is typically low making it desirable to provide a process allowing a higher throughput compared to prior art processes.